Oracle JDBC prefetch: how to avoid running out of RAM
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Using Oracle java JDBC (ojdbc6 11.2.0.4),loading a query with many rows takes forever (high latency environment. This is apparently the default prefetch in Oracle JDBC is default size "10" which requires a round trip time once per 10 rows. I am attempting to set an aggressive prefetch size to avoid this. PreparedStatement stmt = conn.prepareStatement("select * from tablename"); statement.setFetchSize(10000); ResultSet rs = statement.executeQuery();
This can work,but instead I get an out of memory exception. I had presumed that setFetchSize would tell it to buffer "that many rows" as they come in,using as much RAM as each row requires. If I run with 50 threads,even with 16G of -XMX space,it runs out of memory. Feels almost like a leak: Exception in thread "main" java.lang.OutOfMemoryError: Java heap space at java.lang.reflect.Array.newArray(Native Method) at java.lang.reflect.Array.newInstance(Array.java:70) at oracle.jdbc.driver.BufferCache.get(BufferCache.java:226) at oracle.jdbc.driver.PhysicalConnection.getCharBuffer(PhysicalConnection.java:7422) at oracle.jdbc.driver.OracleStatement.prepareAccessors(OracleStatement.java:983) at oracle.jdbc.driver.T4CTTIdcb.receiveCommon(T4CTTIdcb.java:273) at oracle.jdbc.driver.T4CTTIdcb.receive(T4CTTIdcb.java:144) at oracle.jdbc.driver.T4C8Oall.readDCB(T4C8Oall.java:771) at oracle.jdbc.driver.T4CTTIfun.receive(T4CTTIfun.java:346) at oracle.jdbc.driver.T4CTTIfun.doRPC(T4CTTIfun.java:186) at oracle.jdbc.driver.T4C8Oall.doOALL(T4C8Oall.java:521) at oracle.jdbc.driver.T4CPreparedStatement.doOall8(T4CPreparedStatement.java:205) at oracle.jdbc.driver.T4CPreparedStatement.executeForDescribe(T4CPreparedStatement.java:861) at oracle.jdbc.driver.OracleStatement.executeMaybeDescribe(OracleStatement.java:1145) at oracle.jdbc.driver.OracleStatement.doExecuteWithTimeout(OracleStatement.java:1267) at oracle.jdbc.driver.OraclePreparedStatement.executeInternal(OraclePreparedStatement.java:3449) at oracle.jdbc.driver.OraclePreparedStatement.executeQuery(OraclePreparedStatement.java:3493) at oracle.jdbc.driver.OraclePreparedStatementWrapper.executeQuery(OraclePreparedStatementWrapper.java:1491) .... What can I do to still get prefetch but not run out of RAM? What is going on? The closest related item on SO is this:?http://stackoverflow.com/a/14317881/32453 解决方法 Basically,oracle‘s default strategy for later ojdbc jars is to "pre allocate" an array per "prefetch" row that accomodates for the largest size conceivably possible to return from that query. So in my case I had some VARCHAR2(4000) in there,so 50 threads * 3 columns of varchar2‘s * 4000 was adding up to more than gigabytes of RAM [yikes]. There does not appear to be an option to say "don‘t pre allocate that array,just use the size needed." Ojdbc even keeps these preallocated buffers around?between preparedstatements?so it can reuse them. Definitely a memory hog. The fix was to determine the maximum actual column size,then replace the query with (assuming 50 is the max size)? Other things you can do: decrease the number of prefetch rows,increase Xmx parameter,only select the columns you need. Once we were able to use at least prefetch 400 [make sure to profile to see what numbers are good for you,with high latency we saw? I suppose if you wanted to be really aggressive against sparse "really long" rows you might be able to re-query when you run into these [rare] rows. Details ad nauseum?here 最新还有个解决方法，使用ojdbc8，已经是按需分配模式。 （编辑：厦门网） 【声明】本站内容均来自网络，其相关言论仅代表作者个人观点，不代表本站立场。若无意侵犯到您的权利，请及时与联系站长删除相关内容! |
